This invention relates to a copying machine or like image forming apparatus, and more particularly to a control system for controlling a fixing unit provided in the image forming apparatus.
A fixing unit provided in an existing image forming apparatus such as a copying machine includes a fixing roller and a pressing roller. The fixing unit heats a copy sheet at a predetermined fixing temperature while transporting it. whereby fixing a transferred image onto the copy sheet. The fixing roller is provided with a heater for heating the fixing roller and a temperature detector including a thermistor for detecting the temperature of the fixing roller. The temperature of the fixing roller is maintained in a predetermined temperature range by turning on and off the heater at appropriate timings, i.e., on-off controlling the heater in accordance with the temperature detected by the temperature detector.
The fixing unit is activated when the image forming apparatus is powered on. While power is applied to the image forming apparatus, the temperature of the fixing roller is maintained at a predetermined stand-by temperature near the fixing temperature so that a copying operation can be executed any time. Upon start of the copying operation, the temperature of the fixing roller is increased from the stand-by temperature to the fixing temperature, and decreased from the fixing temperature to the stand-by temperature again upon completion of the copying operation.
Recently, a multitude of functions and high performance have been required for an image forming apparatus as an office machine. However, there has also existed an increasing demand for a low cost image forming apparatus having a simplified and small-sized construction. A simplified and small-sized image forming apparatus is realizable by reducing the number of various functions and sensors or making them smaller. Such an apparatus will be power-saving and can be manufactured at a low cost.
For example, the image forming apparatus may have a simple construction by reducing the number of complicated control mechanisms such that copy sheets are manually fed one by one thereto. Further, detection of jam of a copy sheet may be left in an operator's hand. In other words, the operator may discriminates whether the jam has occurred based on whether or not the copy sheet has discharged within a specified period following start of a copying operation. In this way, a jam sensor can be dispensed with. Furthermore, the image forming apparatus can be manufactured at a low cost if various components as well as a main body of the apparatus are formed with low cost resin.
However, promotion of small-sized and low cost image forming apparatus is at variance with the use of material having high heat resistance and makes it difficult to provide a space for releasing the heat and heat releasing members within the apparatus. This requires measures to reduce an amount of heat generated within the apparatus.
Particularly, in the case where a fixing unit provided with a conventional control system is applied in a simple and small-sized image forming apparatus, the temperature of a fixing roller is controlled using a heater all the time while power is applied to the apparatus. Accordingly, it is difficult to suppress a temperature increase within the apparatus.
Further, in this apparatus, the heater is kept energized even during the time while the copying operation is not executed as long as power is applied to the apparatus. Accordingly, the apparatus is obliged to consume more power than necessary.
Moreover, in the case where, for example, power outage occurs while the copy sheet is being transported in the fixing unit causing a copy sheet jam therein, it is not to be discriminated whether the copy sheet has been discharged within a predetermined period. Therefore, the apparatus cannot detect the jam. The copy sheet remaining in the fixing unit is heated by the fixing roller for a long time until a next copying operation, thereby causing problems such as burning of the copy sheet and fire.